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Fabrication, antistatic ability, thermal properties, and morphology of a SPE-based antistatic HIPS composite.

INTRODUCTION

Plastics, with some exceptions which include polyaniline, polyacetylene, polypyrrole, etc., are inherently insulating materials having the electrical conductivity range of [10.sup.-13] to [10.sup.-15] S [cm.sup.-1] (1), (2). However, in many cases, it is desirable to have materials with electrical conductivities. The high electrical charge on the surface of a polymer may cause some problems such as dust contaminations, which in turn may affect both the appearance and performance of the end products, and cause electrical discharge leading to fire or even explosion (3), (4).

To reduce the surface or volume resistivity and enhance the dissipation of the high electrical charge density on the surface of plastics, either antistatic agents or conductive fillers are usually used (5-8), However, the antistatic agents cannot endow the polymer with a persistently antistatic ability. Moreover, the surface resistivity of the polymer is strongly dependent on the relative humidity (RH) of the ambient environment. Conductive fillers (polymers) currently used, present a relatively high cost and some migration problems, which restrict their large scale applications in industry.

Solid-polymer-electrolyte (SPE), owning an ability of ionic conduction, has been widely studied due to its potential applications in rechargeable cells (9-12). However, the conductivity of the reported SPEs only reaches [10.sup.-4] S [cm.sup.-1] at room temperature, which cannot satisfy the requirement of rechargeable cells (more than [10.sup.-3] S [cm.sup.-1]. Therefore, so far, there are almost no reports about SPE applications.

To the best of our knowledge, most of the conducting polymers recently used for the preparation of antistatic composites are intrinsically electron-conductive polymers, such as polyaniline doped with dodecylbenzene sulfonic acid (13). Composites composed of ion-conductive polymers and insulating polymer matrices have been scarcely reported. And thus, the use of an ion-conductive SPE to impart antistatic properties to an insulating polymer seems a relatively novel development and an interesting alternative to the traditional carbon black fillers, because a suitable SPE may not affect the dyeing, processability, and mechanical properties of the polymer matrices. In addition, the application fields of the current SPEs can be also enlarged by this method. However, most of the reported SPEs are prepared by the solvent-based casting method, in which both the polymer matrices and the doping salts are dissolved into volatile solvents. Therefore, it will take too much time and cost more energy for drying to obtain the end products. The evaporation of solvents also brings about severe environmental contamination. Preparation of a new SPE with a high ionic conductivity and without any solvent contamination encourages us to expand our future work. We have previously reported a novel non-solvent method to prepare SPE (14). The conductivity of the prepared SPE is almost equal to that fabricated by the solvent casting method. In this article, we have further developed a solvent-free antistatic composite composed of high impact polystyrene (HIPS) and [LiC1O.sub.4] doped thermoplastic polyurethane (TPU)/poly(ethylene oxide) (PEO). The processability and conductivity of the composites have been evaluated. The composites have also been characterized by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), tensile tests, and scanning electron microscope (SEM) measurements.

EXPERIMENTAL

Materials

Both the polyether (type: 83A10) and polyester (type: 80A) thermoplastic polyurethane were supplied by Bayer Company (Germany). Analytically pure poly(ethylene oxide) (PEO, [M.sub.w] = 6000) was purchased from Kelong Chemical Reagent Co. (Chengdu, China). High impact polystyrene (HIPS, extruded grade) was supplied by Zhonghe Chemical-Plastic Co. (Guangdong, China). Anhydrous [LiClO.sub.4] (99.95%) was analytically pure, and purchased from the National Lithium Material Co. (Chengdu. China). It was dried at 120[degrees]C for 4 hr in a vacuum oven before use. All the other chemicals used were analytically pure, and used as received.

Preparation of the HIPS/RSPE Composites

[LiC1O.sub.4] was mixed with the set amounts of TPU and PEO in a high-speed mixing chamber at room temperature for 2 min, the mixing speed was 1000 rpm and the resulting composites were named raw solid polymer electrolyte (RSPE). Then, the HIPS/RSPE composites were prepared in a Haake torque rheometer equipped with an electrically heated mixing head and two noninterchangeable rotors. The processing temperature, rotor speed, and blending time were set at 160[degrees]C, 60 rpm, and 10 min. respectively.

Characterization

Conductivity Measurement. A surface resistivity meter (ZC46A, Shanghai, China) was used to measure the surface resistivity of the HIPS/RSPE composites at room temperature under the RH of 55%. It was also used to measure the surface resistivity of the sample at different temperatures and RHs. The testing equipment of the surface resistivity is shown in Fig. 1. The diameters of the electrodes of the surface resistivity meter were 25 mm for the anode and 150 mm for the cathode, respectively. The width of the circular cathode was 12 mm. The set voltage of 500 V was used for the measurement. The surface of the sample was cleaned with distilled water, and then dried under vacuum at 30 C for 24 hr before the measurement. A conductive solution mixing of polyethylene glycol ([M.sub.w] = 600), soft soap, and distilled water was used to enhance the contact between the electrode of the tester and the surface of the sample.

[FIGURE 1 OMITTED]

TGA. TGA of the HIPS/RSPE composites and the pure HIPS was carried out in a TA instrument SDT-Q600 thermal analyzer from 25 to 600[degrees]C in a nitrogen atmosphere, with a heating rate of 10[degrees]C [min.sup.-1] and ca. 10 mg of each sample. All the samples were previously heated to 100[degrees]C for 5 min to eliminate the residual water. TGA curves were analyzed by a TA universal analysis program.

DSC Analysis. DSC measurements were performed in a DSC204 (NETZSCH, Germany) analyzer from -50 to 200[degrees]C under the protection of nitrogen, with a heating rate of 10[degrees]C [min.sup.-1] and ca. 10 mg of each sample. The temperature was equilibrated at -50[degrees]C and then scanned from -50 to 200[degrees]C. [T.sub.g] was calculated from the peak of the first derivative of the inflexion in the DSC curve.

Tensile Strength Test. The 1-mm thick sheets of the HIPS/RSPE composites were prepared by using compression molding at 180[degrees]C, and then used for the tensile test by using an Instron4302 at a tensile rate of 100 mm [min.sup.-1]. the dimensions of 25 x 6 x 1 [mm.sup.3] dumbbell samples were prepared for tests.

SEM Analysis. SEM micrographs of the HIPS/RSPE composites and pure HIPS were studied by using a JEOLJSM-5900LV SEM (Japan) set to an accelerated voltage of 20 kV. The SEM samples were gold-sputtered prior to observation.

RESULTS AND DISCUSSION

Surface Resistivity Analysis

Figure 2 shows that the surface resistivity of both the polyether-based and polyester-based HIPS/RSPE composites decreases with the PEO content. In the case of the polyester-based composites, the surface resistivity gradually decreases from [10.sup.11] to [10.sup.9] ohm [sq.sup.-1] when the PEO content in RSPE ranges from 0 to 12 phr. As the PEO content increases from 0 to 4 phr, the surface resistivity of the polyether-based composites sharply decreases from [10.sup.10] to [10.sup.8] ohm [sq.sup.-1]. However, the surface resistivity of the composites slightly decreases with further increasing the PEO content. The addition of PEO to the composites can reduce the surface resistivity of the sample about two orders of magnitude, because PEO with a relatively low molecular weight can act as a plasticizer and thus significantly enhance the mobility of TPU soft segments. The improvement of the mobility of molecular chains implies the conductivity enhancement for a typical SPE. At the same PEO content, the surface resistivity of the polyether-based composites is lower than that of the polyester-based composites, due to the fact that the ether oxygen groups originating from the soft segments of the polyether TPU are able to effectively dissociate [LiClO.sub.4] to obtain more free lithiumcations than the polyester TPU. The surface resistivity of both the polyether-based and polyester-based HIPS/RSPE composites is below [10.sup.10] ohm [sq.sup.-1] as the PEO content reaches 4 phr, indicating that these composites can be used in the antistatic packaging field.

[FIGURE 2 OMITTED]

As seen in Fig. 3, the surface resistivity of the HIPS/RSPE composites decreases with the RSPE content. It remains almost a constant up to 2 wt%. When the amount of RSPE increases from 2 to 20 wt%, the surface resistivity of the HIPS/RSPE composites dramatically reduces from [10.sup.15] to [10.sup.10] ohm [sq.sup.-1] for the polyester-based composites and from [10.sup.15] to [10.sup.9] ohm [sq.sup.-1] for the polyether-based composites, respectively. However, the surface resistivity of the composites slightly reduces with further increasing the RSPE content, indicating that there have been percolation thresholds (PTs) in such antistatic HIPS/RSPE composites, and the PTs of the polyether-based and polyester-based HIPS/RSPE composites are at about 5 and 10 wt%, respectively. The surface resistivity of the composites is too high to satisfy the requirement of antistatic applications when the RSPE content is lower than the PTs, due to the discontinuous ion-conductive phase of RSPE. The surface resistivity of the composites sharply reduces to [10.sup.10] ohm [sq.sup.-1] as the RSPE content surpasses the PTs, because the continuous ion-conductive phase is able to thoroughly form the conductive tunnels or networks in the polymeric matrix (15), in which the lithium cations can freely transfer under the setting electric field. More RSPE contents only increase the number of ion-conductive tunnels, because the ratio of EO/[Li.sup.+] keeps a constant. It is the reason why the surface resistivity of the composites remains almost a constant when the RSPE content is over 20 wt%.

[FIGURE 3 OMITTED]

The antistatic capacity of traditional antistatic-agent-based polymers is deteriorated with decreasing the RH. At a low RH, e.g., 12%, there are no enough lone pairs, deriving from the oxygen atoms in water molecules, to dissociate the ionic antistatic-agents or to coordinate with the oxygen atoms by hydrogen bonds, and thus the surface resistivity of the composites is significantly increased. Figure 4 shows the influence of the RH on the surface resistivity of the prepared HIPS/RSPE composites. The surface resistivity of both the polyether-based and polyester-based HIPS/RSPE composites gradually increases one order of magnitude when the RH reduces from 55% to 12%, implying that such HIPS/RSPE composites can maintain a good antistatic ability even at the RH of 12%. Moreover, the surface resistivity of the composites is found to abruptly increase about half an order of magnitude at the RH of 30%, showing that the RH of 30% is probably the critical value of the coordination effects between TPU/PEO and water molecules. In the case of the high RH, the water molecules in the ambient environment are able to coordinate with the soft segments of TPU and PEO by hydrogen bonds, which will dissociate [LiClO.sub.4] effectively. At the low RH, there are few water molecules to coordinate with TPU or PEO molecular chains. However, the TPU and PEO molecular chains themselves are also able to dissociate [LiClO.sub.4] to get lithiumcations. Therefore, the addition of such RSPE to the HIPS host is able to endow the HIPS matrix with a permanently antistatic ability instead of the temporarily antistatic property of the traditional antistatic agents.

[FIGURE 4 OMITTED]

The effect of temperature on the surface resistivity of the HIPS/RSPE composites is shown in Fig. 5. The ion-conductive nature of a typical SPE is seen in the prepared HIPS/RSPE composites. The surface resistivity gradually decreases with temperature, implying that the conductivity of the HIPS/RSPE composites is attributed to the formation of a continuous ion-conductive RSPE phase. The dependences of the surface resistivity of the polyether-based and polyester-based HIPS/RSPE composites on the temperature are in good agreement with the Arrhenius and VTF equation (16-19), respectively. This is probably due to the different mobility of the TPU molecular chains correlated to the glass transition temperature ([T.sub.g]), and which will be discussed in the DSC analysis later. Figure 5 also illustrates that the fabricated HIPS/RSPE composites will keep their good antistatic abilities at the temperature range of 20-90[degrees]C, showing the fact that such composites can be employed to more application fields where the low surface resistivity under a high temperature is needed.

[FIGURE 5 OMITTED]

Thermal Processability of the HIPS/RSPE Composites

Thermal processability is very important for thermoplastic polymers as an excellent processability usually means a continuous stable fabrication of the end products. All the conventional SPEs are commonly prepared by the solvent-based methods, and thus, the melt-based non-solvent method seems to be a novel development. Figures 6 and 7 are the fusion curves of the polyether-based and polyester-based HIPS/RSPE composites with different PEO contents blended in a Haake torque rheometer, respectively. As can be seen from the figures, three specific points are observed from each curve. The first point, A, stands for the composites loading. The second point, B, is generated due to the balance between sample loading and the driving force of free material flow, i.e., unmelted HIPS. The third point, C, is generated because of the material compaction and onset of fusion (20). When the blending time surpasses C, the temperature gradually increases to about 160[degrees]C due to the release of some thermal energy developed by the friction of the composites. With increasing the PEO content, the fusion time of both the polyether-based and polyester-based HIPS/RSPE increases obviously. However, all the HIPS/ RSPE composites can be also successfully fabricated in a given time. The higher PEO content results in decreasing the melt viscosity of the composites, therefore, the time period between the loading point and the stopping point increases. This is probably accounted for that the soft molecular chains of PEO as the plasticizer effectively decreases the friction of the relatively rigid HIPS/RSPE composites. At the same PEO content, the polyester-based HIPS/RSPE composites can be more easily prepared than the polyether-based HIPS/RSPE composites, resulting from the higher melt viscosity of the polyester TPU. More PEO contents in the HIPS/RSPE composites imply a better antistatic ability because there have been more PEO molecular chains to dissociate [LiClO.sub.4] to get more free lithium cations. However, both the polyether-based and polyester-based HIPS/RSPE composites are unable to be prepared in the experimental time at the high PEO content.

[FIGURE 6 OMITTED]

TGA Results

Figure 8 shows the TGA curves of the HIPS/RSPE composites with different PEO contents. With the exception of pure HIPS, all the rest composites own two thermal degradation onset temperatures. For the polyether-based HIPS/RSPE composites, the first weight loss stage slightly increases with the PEO content, and its temperature range is between 244 and 326[degrees]C. This is assigned to the thermal decomposition of PEO. The second weight loss stage at the temperature range of 411-447[degrees]C is assigned to the thermal degradation of HIPS. For the polyester-based composites, both the first and the second weight loss stages are increased in comparison with those of the polyether-based composites, due to the fact that the mobility of molecular chains of the polyester TPU is lower than that of the polyether TPU. All the composites present a sharp weight loss above 410[degrees]C, corresponding to the degradation of both the HIPS matrix and TPU main chains. Figure 8 also shows that the residue of the sample slightly increases with the PEO content. The residues of the polyether-based and polyester-based HIPS/RSPE composites reach 2.5 and 3.9 wt%, respectively. On the basis of the earlier discussion, it is concluded that the HIPS/RSPE composites can be fabricated without any decompositions when the processing temperature is below 250[degrees]C, indicating that such composites will keep their excellent antistatic properties after normal processing methods such as Haake mixing as well as extrusion.

[FIGURE 8 OMITTED]

DSC Results

Glass transition temperature ([T.sub.g]) is one of the most important factors which directly influences the conductivity of SPE. Lower [T.sub.g] and higher mobility of the polymer molecular chains usually implies a better conductivity for SPE (21), (22). DSC curves for the pure HIPS and HIPS/RSPE composites arc compared in Fig. 9. In the case of the pure HIPS (Fig. 9a), a typical [T.sub.g] at around 100[degrees]C is observed. The [T.sub.g] of HIPS/RSPE at the higher temperature of 100[degrees]C seems almost unchanged after the addition of RSPE, indicating a moderate miscibility between the HIPS host and RSPE, and this will be helpful for the formation of RSPE conductive networks in the HIPS matrix. However, in the case of the polyether-based HIPS/RSPE composites (Fig. 9b,c), when the PEO content ranges from 4 to 10 phr, the [T.sub.g] originating from the soft segments of TPU decreases from about 0[degrees]C to a lower temperature, which cannot be observed at the experimental temperature range. It indicates that a better antistatic capacity of the HIPS/RSPE composites can be obtained when the PEO content considerably increases, which is in good accordance with the surface resistivity analysis in the earlier section. Similar variety tendency of [T.sub.g] is also observed in the DSC curves of the polyester-based HIPS/RSPE. However, the [T.sub.g] of the polyester-based HIPS/RSPE at about 25[degrees]C is higher than that of the corresponding polyether-based composites, and this is the possible reason why the conductivity (surface resistivity) of the polyether-based composites is superior than that of the polyester-based composites at the same PEO content.

[FIGURE 9 OMITTED]

Mechanical Properties Results

Conventional SPEs are commonly considered as materials with bad mechanical properties, especially, when SPEs are blended with low molecular weight plasticizers. Therefore, in this article, it is necessary to find out the influence of PEO with a low molecular weight on the mechanical properties of the HIPS/RSPE composites, because a good additive, including inorganic and organic compounds, will not decrease the mechanical properties of the host polymer. The tensile strength and elongation at break of different HIPS/RSPE composites are shown in Fig. 10. It reveals that the tensile strength of both the polyether-based and polyester-based HIPS/RSPE composites slightly changes (approximate to 20 MPa) with the PEO content. The elongation at break of the composites increases in a given PEO content, however, it decreases when the PEO content surpasses the critical value (CV). The CVs of the polyether-based and polyester-based HIPS/RSPE composites are at about 6 and 4 phr, respectively. Increasing the PEO content from 0 phr to the CVs enhances the elongation at break of the composites from 5.16 to 16%. The increment of the elongation at break before the CVs is attributed to that the flexible molecular chains of PEO as a plasticizer effectively decreases the interactions between TPU and HIPS matrix, which is propitious to disperse the TPU-based SPE networks into the HIPS matrix. RSPE elastomer as a continuous phase evenly disperses in the HIPS matrix, and thus significantly enhances the toughness of the composites. More PEO contents decrease the elongation at break due to the fact that the TPU content in the composites also decreases at the same time. The earlier discussion indicates that the addition of PEO to HIPS/RSPE can effectively improve the antistatic property of the composites, and will not decrease the mechanical properties of the HIPS/RSPE host, showing that such mechanical properties of the composites can satisfy the corresponding requirement of the packaging application in electronic devices.

[FIGURE 10 OMITTED]

SEM Investigation

SEM of the pure HIPS and various HIPS/RSPE composites is shown in Fig. ll(a-d). In Fig. 11a, the surface of the pure HIPS shows a relatively rough and uneven topography. Some microparticles (about 2 [micro]m) are observed. As shown in Fig. 11b, the [LiClO.sub.4] doped TPU is found to have a moderate miscibility with the HIPS host, the visible phase segregation between the HIPS matrix and the ion-conductive polymer has not been observed. The addition of PEO to the composites is propitious to enhance the dispersion of the RSPE into the HIPS matrix. As can be seen from the SEM of the surfaces etched by 1,4-dioxane (Fig. 11c,d), typical tunnels topography of the TPU doped with alkali salt is seen in the surface morphology of the HIPS/RSPE composites. These tunnels formed by the addition of RSPE are helpful for the conductivity improvement of the HIPS/RSPE composites. In addition, these veins in the composites without PEO are large and dispersed inhomogeneously, which will restrict the decrease of the surface resistivity of the composites. Morphological study also reveals that with increasing the PEO content, more RSPE tunnels and networks in the HIPS/RSPE composites are formed, implying the fact that PEO is able to further enhance the conductivity (decrease the surface resistivity) of the HIPS/RSPE composites.

[FIGURE 11 OMITTED]

CONCLUSIONS

Permanently antistatic HIPS/RSPE composites with the surface resistivity range of [10.sup.[8-10]] ohm [sq.sup.-1] can be successfully prepared by blending the solvent-free [LiClO.sub.4] doped polyurethane-based SPE with the HIPS host in a Haake torque rheometer. The surface resistivity of the HIPS/RSPE composites is below [10.sup.10] ohm [sq.sup.-1] and adequate for the antistatic applications when the PEO content reaches 4 phr. The conductivity of the HIPS/RSPE composites is originated from the formation of the continuous ion-conductive RSPE phase in the HIPS matrix. The dependences of the surface resistivity of polyether-based and polyester-based composites on the temperature are fitted to the Arrhenius and VTF equation, respectively.

The surface resistivity of both the polyether-based and polyester-based HIPS/RSPE composites gradually increases one order of magnitude when the RH reduces from 55% to 12%, implying a good antistatic ability of such HIPS/RSPE composites at the low RH.

With increase of the PEO content, the fusion time of both the polyether-based and polyester-based HIPS/RSPE composites increases considerably. The HIPS/RSPE composites can be fabricated without any decompositions when the processing temperature is below 250[degrees]C, indicating that such composites will keep their excellent antistatic properties after normal processing methods.

The tensile strength of both the polyether-based and polyester-based HIPS/RSPE composites is not remarkably changed with the PEO content. It can satisfy the corresponding requirement of the antistatic packaging field in electronic devices.

REFERENCES

(1.) I. Krupa, G. Mikova, I. Novak, I. Janigova, Z. Nogellova, F. Lednicky, and J. Prokes, Eur. Polym. J., 37, 1813 (2001).

(2.) I. Novak, I. Krupa, and I. Chodak, Eur. Polym. J., 39, 585 (2003).

(3.) W. Thongruang, R.J. Spontak, and C.M. Balik, Polymer, 43, 2279 (2002).

(4.) I. Chodak, M. Omastova, and J. Dionteck, J. Appl. Polym. Sci, 82, 1903 (2001).

(5.) G. Chen, C. Wu, W. Weng, D. Wu, and W. Yan, Polymer, 44, 1781 (2003).

(6.) G.H. Chen, D.J. Wu, W.G. Weng, and W.L. Yan, J. Appl. Polym. Sci., 82, 2506 (2001).

(7.) Y.X. Pan, Z.Z. Yu, Y.C. Ou, and G.H. Hu, J. Polym. Sci. Part B, Polym. Phys., 38, 1626 (2000).

(8.) M. Pluta, M. Alexandre, S. Blacher, P. Dubois, and R. Jerome, Polymer, 42, 9293 (2001).

(9.) D.E, Fenton, J.M. Parker, and P.W. Wright, Polymer, 14, 589 (1973).

(10.) P. Lightfoot, J.L. Nowinski, and P.G. Bruce, J. Am. Chem. Soc., 116, 7469 (1994).

(11.) G.S. MacGlashan, Y.G. Andreev, and P.G. Bruce, Nature, 398, 792(1999).

(12.) F. Croce, G.B. Appetecchi, L. Persi, and B. Scrosati, Nature, 394, 456 (1998).

(13.) R.M. Cristiane and D.P. Marco-Aurelio, Eur. Polym. J., 41, 2867 (2005).

(14.) J.L. Wang, W.Q. Yang, and J.X. Lei, J. Electrostal., 66, 627 (2008).

(15.) I. Novak, I. Krupa, and I. Janigova. Carbon, 43, 841 (2005).

(16.) M.J. Reddy and P.P. Chu, J. Power Sources, 135, 1 (2004).

(17.) P. Basak and S.V. Manorama, Solid State Ionics, 167, 113 (2004).

(18.) A.M. Stephan, S.G. Kumar, N.G. Renganathan, and M.A. Kulandainathan, Eur. Polym. J., 41, 15 (2005).

(19.) M. Marzantowicz, J.R. Dygas, and F. Krok, Electrochim. Acta., 53, 7417 (2008).

(20.) C.H. Chen. H.C. Li, C.C. Teng. and C.H. Yang, J. Appl. Polym. Sci., 99, 2167 (2006).

(21.) Z. Stoeva, I. Martinlitas, E. Staunton, Y.G. Andreev, and P.G. Bruce, J. Am. Chem. Soc, 125, 4619 (2003).

(22.) F. Croce, G.B. Appetecch, L. Persi, and B. Scrosati, Nature, 394, 456 (1998).

Wanqing Yang, Jiliang Wang, Jingxin Lei

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China

Correspondence to: Jingxin Lei; e-mail: jxlei@scu.edu.cn

DOI 10.1002/pen.21578

Published online in Wiley InterScience (www.interscience.wiley.com).

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Author:Yang, Wanqing; Wang, Jiliang; Lei, Jingxin
Publication:Polymer Engineering and Science
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Date:Apr 1, 2010
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